Advanced-stage prostate cancer often metastasizes to bone but becomes incurable due to poor biodistribution of intravenously administered anticancer drugs within bone. Bisphosphonates are currently used to reduce the risk of skeleton-related events and to ameliorate bone pain, but they do not improve survival. Injected drugs or drug-loaded nanocarriers conjugated to bone-seeking agents remain inefficient in treating bone metastasis. In this proposal, as a therapy for bone metastasis, we explore a new route for nanoparticle (NP)-mediated drug delivery to bone - the intracellular clefts between endothelial cells in bone marrow. PEGylated NPs, because of their hydrophilic surface, remain in the circulation rather than efficiently extravagating through the fenestrations in bone-marrow capillaries. Our novel approach has been to design non-PEGylated NPs and rationalize their characteristics (size, charge, and surface composition) so that following their intravenous administration, these NPs do extravagate through the openings of the marrow's sinusoidal capillaries. In a preliminary study using a PC-3M-luc cell-induced osteolytic intraosseous mouse model of prostate cancer, (a) after intravenous injection, these NPs demonstrated focal accumulation in bone marrow within metastatic sites and (b) a single dose of paclitaxel-loaded NPs significantly inhibited the progression of bone metastasis and completely prevented bone loss In this proposal, our objective is to evaluate these NPs in a clinically relevant model of bon metastasis, induced by intracardiac injection of PC-3M-luc cells. We will test the efficacy of denosumab in NPs (DNmb- nano). DNmb is a monoclonal antibody that binds to receptor activator of nuclear factor-?B ligand (RANKL). Over expression of RANKL in the bone microenvironment drives the vicious destructive cycle of progression of bone metastasis and bone resorption. We will also explore DNmb as a targeting ligand against RANKL and use docetaxel loaded in NPs (TXT-nano), as TXT has proven more potent than paclitaxel for treating prostate cancer. We hypothesize that using our NPs, which effectively localize to bone, and the combination of DNmb and TXT with their complementary mechanism of action could inhibit progression of bone metastasis and prevent bone loss. The specific aims are: AIM 1: Delineate the parameters critical for efficient localization of NPs to metastatic sites in a prostate tumor model of advanced-stage bone metastasis. AIM 2: Evaluate the efficacy of the combination of docetaxel- and denosumab-loaded NPs (TXT-nano and DNmb-nano) to inhibit progression of bone metastasis and the resulting bone loss. AIM 3: Determine the pharmacokinetics and pharmacodynamics of drug distribution and tolerance to drug treatment by use of NPs. Impact: Since bone is a common site for metastasis in a number of human cancers, with devastating consequences, an effective drug-delivery strategy could potentially have considerably broader implications.